Richard Suchenwirth 2002-11-07 - Tcl's upvar command ties a local variable in a proc to another in a higher scope in the call stack - typically the caller's (upvar 1). This way, references (like pointers in C, or literally "call by name") can be implemented. The Tcl Style Guide recommends "Name" as suffix for upvar-ed arguments, e.g.
proc example {varName value} {
upvar 1 $varName var
...}which is a good convention for readable code. However, after a hard day of adjusting *const** references in C++, frustration with that baroque language brought me to think of other ways in the freeest language I know - Tcl. I did not want to repeat the *const** experience, but simple references can be sugared by overloading the proc command.
For this breakfast fun project, I chose the slightly different name pruc ("procedure with upvar components"). It is called just like proc, with the added behavior that arguments declared with prefix * or & are implicitly upvar-ed to a local variable without that prefix, before the original proc body. Simple enough, but again a few lines of code have changed the face of our language considerably: }
proc pruc {name argl body} {
set prefix ""
foreach {upvar var} [regexp -all -inline {[*&]([^ ]+)} $argl] {
append prefix "\nupvar 1 \${$upvar} $var;"
}
proc $name $argl $prefix$body
}#------------- Testing:
pruc demo {*i j} {
set i $j
}
puts "[demo foo 42]/$foo"
pruc demo2 {j &i} {
set i $j
}
puts "[demo2 43 bar]/$bar"
puts "generated body: [info body demo2]"if 0 {...which shows on stdout:
42/42
43/43
generated body:
upvar 1 ${&i} i;
set i $j
}See Pass by reference for an earlier alternative, and use_refs
escargo 15 Dec 2005 - I wonder if you thought about a different policy: When an argument name ends in "Name" (e.g., varName), automatically insert upvar $varName var into the synthesized procedure. That would follow the Style Guide.
KPV wouldn't work for me because I often have parameters with names like imageName, fileName and widgetName.
AMG: I wasn't aware that this Name suffix style had been codified, so I made up my own convention: I usually suffix "name" argument names with "_var". For instance, if I wrote a proc that as a side effect would set some variable in the caller's frame to the name of an image, I'd call that parameter imagename_var. To me that name suggests that the caller should pass the name of the variable storing the image name.